The Critical Role of Soil Microbiota to Sustainable Agriculture: Quantifying short-term microbial and vegetation feedback to intensive grazing.

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Commodities

• Agronomic: grass (misc. annual)
• Animals: bovine

Practices

• Animal Production: grazing management, grazing - continuous, grazing - rotational, pasture fertility, rangeland/pasture management, stocking rate
• Soil Management: soil analysis, soil microbiology, soil quality/health

Grazing is known to have long-term effects on soil biology and vegetation, but little is known about how grazing impacts the soil microbial community on a short timescale.Understanding how above-and below-ground processes respond to grazing disturbance is an important step to understanding how ecosystem services shift following grazing impacts. To address this knowledge gap, a grazing trial was conducted over two growing seasons, and soil microbial community data, vegetation biomass, and soil labile nutrients were quantified immediately after through four weeks following different grazing severities. This research is the first to show that both high-intensity, short-duration grazing and low-intensity, medium-duration grazing cause detectable changes in the soil microbial community and labile soil nutrients as soon as 24 hours after grazing. Additional research questions are addressed by summarizing the current state of recent grazing research utilizing molecular methods, and by testing a novel method to use ground-level spectral imagery to measure vegetation biomass opposed to traditional methods. Overall, this research contributes valuable knowledge to the field of soil microbial ecology and land management. The results and management recommendations were disseminated to producers via Extension outreach and to undergraduate students at University of Wyoming as part of soil health curriculum. To convey results to the scientific community, a poster was presented at the Ecological Society of America conference, and we will submit manuscripts to peer-reviewed journals.

Research Questions:

1. Do soil microbial dynamics differ in response to intensive grazing regimes as opposed to light grazing and grazing exclusions?
2. Is there an immediate response of the soil microbial community to high-intensity or low-intensity grazing compared to grazing exclusions?
3. What is the effect of intensive defoliation on vegetation recovery growth?
4. Will an increase in microbial dynamics affect vegetation recovery growth?

We hypothesized different soil microbiological responses to different grazing management strategies. To test this hypothesis, we conducted a grazing trial over two growing seasons that compared high-intensity,short-duration grazing to low-intensity, medium-duration grazing and a no-grazing control in an irrigated meadow dominated by introduced forage species. Vegetation biomass, soil labile nutrients and the soil microbial community activity, diversity, and function were assessed before grazing and 24 hours through four weeks immediately after grazing. We utilized modern high-throughput molecular techniques to analyze the soil microbial community, including extracellular enzymatic assays and 16S and ITS amplicon sequencing. This is the first study that assesses the immediate response of the soil ecosystem to grazing disturbance.

While exploring the potential use of spectral imaging to replace traditional methods of measuring aboveground biomass, we found that while the method has potential, the relationship between red, blue, and near-infrared light wavelengths and vegetation biomass is not valid in high-production systems, i.e. with a dried vegetation biomass over 2000 kg/acre. Ground-level spectral imaging with commercially available cameras has potential to replace time-costly traditional methods of monitoring forage quantity, but a more available workflow is needed to make data analysis easier for land managers, and a different spectral index is needed that does not saturate at high vegetation biomass and canopy cover. This is beneficial to land managers who are interested in monitoring small-scales changes in vegetation, which is highly recommended when utilizing high-intensity grazing methods.

We found that labile soil C and N increased following the low-density grazing while C-cycling extracellular enzymatic activities increased in response to high-density grazing but total extracellular enzymatic activity profiles were strongly affected by sampling time. The soil fungal community structure was strongly affected by the interaction of sampling time and grazing treatment, while the soil bacterial community was largely resilient to change. We found evidence of seasonal influences on soil chemical and microbial parameters, even over the total sampling time of five weeks during the growing season. These results highlight the complexity of soil microbiota and the difficulty of translating ecological dynamics into real-world management recommendations. Although our results showed that high-intensity, short-duration grazing has an immediate impact on soil microbial dynamics, it is difficult to ascribe a management quality to those changes, i.e. if the changes are beneficial to ecosystem services that matter to producers. However, these results confirm that producers have an ability to impact soil biological functioning with management changes which holds importance for producers that may be interested in changing grazing styles on irrigated pastures.

This research found that the soil microbial community and soil labile nutrients respond as soon as 24 hours following grazing, and seasonal and spatial heterogeneity has a large impact on soil microbial activity and functioning. These results have implications for future experimental designs and future research and illustrate a need for research on better functional understanding of the soil microbial community.This study is the first to detect pulses in soil microbial activity,diversity, and function within 24 hours of grazing disturbance, but future research is needed to assign functional significance to taxonomic shifts. This need is reflected in current research findings on grazing impacts on the soil microbial community, which indicate that climate, soil type, and grazing intensity drive the severity of impacts on the soil microbial community. In order to translate research to management recommendations, research should begin to link phylogeny to function. Soil microbial richness and diversity is relatively resilient to grazing impacts,but the structure of the community can respond, either on very short timescales as we have shown here, or on longer time scales, as has been previously documented.

1. Quantify the effects of high-intensity grazing compared to conventional grazing on soil physical, chemical, and biological parameters over a growing season.
2. Detect the flux of microbial community dynamics immediately following high-intensity grazing events, contrasted to fluxes that occur during conventional, low-intensity grazing.
3. Document the immediate response of vegetation to homogenous, intensive defoliation and to heterogeneous, moderate defoliation.
4. Synthesize the interactions of above-and below ground processes in response to high-intensity, short-duration grazing.